1. Field of the Invention
This invention relates to a reference position detecting device and more particularly to a horizontal position detecting device for correctly positioning a wafer surface or a surface of a body to be inspected at a position vertical to the optical axis of an objective lens.
2. Description of the Prior Art
In general, for an exposure device of reduction projection type used for making an integrated circuit, a projecting objective lens having a large numerical aperture (N.A.) has been used so that an allowable focus range was very small. Therefore unless the exposing region of a wafer is maintained at an exact vertical position relative to the optical axis of a projecting objective lens, it was not possible to make a clear pattern exposure over the whole exposing region. The whole of a wafer could be aligned in nearly vertical position by detecting three points on the surface of the wafer using a separately arranged auto-focus mechanism. However, as a wafer becomes larger or a wafer is made of a new material such as gallium-arsenic instead of silicon, evenness of the surface of the wafer per se becomes unstable, so that it is necessary to detect vertical positions of the respective parts of the wafer. Therefore, each time the exposure and chemical treatment have been made, the deformation of the wafer is increased and it becomes indispensable to detect correct horizontal positioning of the exposure regions.